This invention relates to PEM/SPE fuel cells, and more particularly to a method of making electrodes and combination membrane electrode assembly.
Electrochemical cells are desirable for various applications, particularly when operated as fuel cells. Fuel cells have been proposed for many applications including electrical vehicular power plants to replace internal combustion engines. One fuel cell design uses a solid polymer electrolyte (SPE) membrane or proton exchange membrane (PEM), to provide ion exchange between the anode and cathode. Gaseous and liquid fuels are useable within fuel cells. Examples include hydrogen and methanol, with hydrogen being favored. Hydrogen is supplied to the fuel cell""s anode. Oxygen (as air) is the cell oxidant and is supplied to the cell""s cathode. The electrodes are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode. A typical fuel cell is described in U.S. Pat. No. 5,272,017 and U.S. Pat. No. 5,316,871 (Swathirajan et al.).
Important aspects of a fuel cell include reaction surfaces where electrochemical reactions take place, catalysts which catalyze such reactions, ion conductive media, and mass transport media. The cost of power produced by a fuel cell is, in part, dependent on the cost of preparing electrodes and membrane electrode assemblies (MEA). The cost of power produced by a fuel cell is greater than competitive power generation alternatives, partly because of the cost of preparing such electrodes and MEAs. However, power produced from hydrogen-based fuel cells is desirable because hydrogen is environmentally acceptable and hydrogen fuel cells are efficient.
Therefore, it is desirable to improve the manufacture of such assemblies and to improve the cost and render fuel cells more attractive for transportation use.
The present invention provides a method of making membrane electrode assemblies (MEAs) which, as set forth hereafter, reduces electrode defects, particularly the phenomenon known as mud-cracking. The invention provides two approaches to prepare an electrode, a pretreatment approach and a post-treatment approach. The method utilizes porous substrate for drying slurries applied thereto to form an electrode. In one aspect, the application of the slurries involves a selective use of relatively wetting and non-wetting solvents to regulate the drying of the slurries to form an electrode film.
In the pretreatment approach, a porous support substrate is coated with a wetting solvent such that the solvent is imbibed into the pores. A slurry is formed including an ionically conductive material, a catalyst supported on an electrically conductive material, and a solvent that is non-wetting to the porous substrate. The slurry is well mixed and applied as a layer to the surface of the porous support substrate and dried to form a film. The film is applied to a membrane, and heat and pressure are applied to form a membrane electrode assembly. Advantageously, this method controls the drying to form a more robust electrode by preventing electrode shrinkage and subsequent cracking of the electrodes.
In the post-treatment approach, the process begins by forming a slurry containing an ionically conductive material, a catalyst, and a solvent that is wetting to the porous substrate. The slurry is well mixed and coated onto a surface of the substrate and dried to form a layer. The dried layer is overcoated with a solution of an ionomer and a solvent that is non-wetting to the substrate. This method, in addition to controlling the drying, advantageously provides for more precise control of ionomer content in the electrodes. This occurs because the ionomer is added to the electrode with a solvent that will not penetrate the porous substrate and therefore ionomer is not leached out of the applied layer and into the substrate.
An important feature of the present invention is the use of a porous substrate to prepare a membrane electrode assembly (MEA) sequentially comprising a first electrode, an ionically conductive membrane, and a second electrode. The porous substrate is releasably attached to respective electrodes and this facilitates the handling of the electrodes and the MEA during fabrication steps. The porous substrate enhances the control of drying of the electrode by permitting vapors to pass through the substrate, and provides for better bonding of the electrode to the membrane by permitting passage of water vapor through the porous substrate generated during hot-pressing.